The present invention relates generally to electrical switches, and more particularly to non-contact piezoelectric switches and interface circuits therefor.
Switches are increasingly being used in a variety of application and particularly in groups for the control of household appliances, industrial control applications, and alphanumeric keyboards, for example. For these applications, a small size low cost, high reliability switch, which is relatively insensitive to outside electromagnetic interference, is desired.
Conventional metal contact switches have, of course, been used for some time. With these conventional metal contact switches, the contact resistance tends to increase with age, and also is affected by external environmental conditions, such as temperature and moisture. This tends to create long-term reliability problems due to increasing contact resistance. An example of a multiple contact switching apparatus is U.S. Pat. No. 3,240,885 (Grunfelder, et al.) Multiple Switching Apparatus, issued Mar. 15, 1966.
Non-contact switches have also existed for some time. An example of such a non-contact touch switch would be a capacitance sensor. However, in order for capacitance sensors to work properly, the touch surface must be kept clean from dirt, grease, or other contaminants. Further, the capacitance sensor might not work if the operator is wearing gloves. A still further disadvantage with capacitance sensors is that most are not readily configured into a matrix array, and further that they provide no tactile sensation of switch activation.
To solve some of these problems, non-contact piezoelectric switches have been developed. In general, an impact imparted upon a piezoelectric ceramic generates a voltage which may be utilized to indicate switch closure. Examples of these piezoelectric switches are U.S. Pat. No. 4,078,187 (Kanisawa) Piezoelectric Switching Device, issued Mar. 7, 1978; U.S. Pat. No. 3,648,279 (Watson) Multielectrode Transducer Element, issued Mar. 7, 1972; and U.S. Pat. No. 3,303,290 (Suloway) Signaling Arrangements Employing Piezoelectric Devices. An improved type of non-contact piezoelectric switch has replaced the piezoelectric ceramic material with a piezoelectric polymeric material; for example, polyvinylidene fluoride. Some of these piezoelectric polymeric switches have been arranged in keyboard type arrangements. An example is U.S. Pat. No. 3,935,485 (Yoshida, et al.) entitled "Piezoelectric Key Board Switch" issued Jan. 27, 1976, which includes a single layer of piezoelectric polymer film having electrodes on both surfaces. A second example is U.S. Pat. No. 3,940,637 (Ohigashi, et al.) Polymeric Piezoelectric Key Actuated Device, issued Feb. 24, 1976, which also includes a single piezoelectric polymer film having electrodes on both surfaces thereof. Other examples of piezoelectric non-contact switches include U.S. Pat. No. 3,382,338, (Arseneault, et al.) Push Button Actuator for Elastomeric Switch, issued May 7, 1968; U.S. Pat. No. 3,798,473 (Murayama, et al.) Polymer-Type Electro-Acoustic Transducer Element, issued Mar. 19, 1974; and U.S. Pat. No. 3,832,580 (Yamamuro, et al.) High Molecular Weight, Thin Film Piezoelectric Transducers, issued Aug. 27, 1974.
A disadvantage to the above-described apparatus, especially when configured in a multi-layer piezoelectric polymer film structure is that the devices are not shielded and are therefore, sensitive to outside electromagnetic disturbances. Since the signal generated by the strained piezoelectric polymer film material is relatively small the device is particularly sensitive to outside interference.
Since the signal pulse generated by the strained piezoelectric polymeric material is relatively small, it therefore needs to be amplified by associated electronic circuitry in order to produce a signal which may be transmitted to other electronic components to indicate switch closure. An example of such associated electrical circuitry is illustrated in U.S. Pat. No. 4,078,187, (Kanisawa, et al.) Piezoelectric Switching Device, issued Mar. 7, 1978. A disadvantage in this electrical circuit, however, is that it is not as sensitive as possible, and its threshold is not readily adjustable.